The field of the invention is industrial control systems, and, particularly, systems which include a plurality of processors, such as programmable controllers, numerical controls and process controls, which are interconnected by a communications network.
There is a wide variety of communications networks which have been employed to interconnect industrial control processors. Such networks include a communications media such as coaxial cable, fiber optics or twisted wire pairs, and they may have any one of a number of topologies, such as a star, multidrop, or ring. Regardless of the media used or the topology of the network, a control scheme is required to provide an orderly transfer of information from one station to another on the network. Many such control schemes are known in the art and attempts have been made to develop standards which enable manufacturers to design equipment which will work on networks that comply with the standard. For example, the Institute of Electrical and Electronic Engineers (IEEE) has developed IEEE Standard 802.4, entitled "Token Passing Bus Access Method and Physical Layer Specification" which sets forth the basic operating procedures to be followed by compliant stations.
The concept of distributed processing, or the control of real time devices with a plurality of interconnected control processors, is an old one. Such distributed processing has been successfully implemented in many applications where "real time" is measured in seconds, minutes, or even hours, and the communications delay between the distributed processors has not been a limiting factor. There are few successful applications, however, where the distributed processors are controlling devices which operate in real times measured in milliseconds, and data from one processor must be conveyed to another at rates commensurate with the operation of such real time devices.
In U.S. Pat. No. 4,747,100, which was filed on Aug. 11, 1986 and is entitled "Improved Token Passing Network," a local area network suitable for peer-to-peer communications between station level controllers is described. This local area network is based on the IEEE 802.4 token passing standard, but it includes unique extensions which make it particularly useful for high speed distributed processing applications.
One of the difficulties with any prior control system which employs distributed control processors is the level of skill and the large effort required to configure, or program, the system. Typically, each control processor in the system must be programmed in its own language to carry out its specific control tasks. In addition, each control processor must be separately programmed in its own language to provide data to other control processors in the system and to receive data from other such control processors. Such programming involves instructions which expressly send or receive data through the local area network linking the control processors. This task requires that the programmer know how to use the network protocol and that he use it in such a way as to provide the necessary data between processors at the required rate without overloading, or "hogging", the network.